The present invention generally relates to a rearview mirror for a vehicle and to microwave receivers and antennas. More specifically, the present invention relates to electrochromic rearview mirror assemblies and control systems, vehicle navigation systems, satellite-to-vehicle communuications, vehicle compass systems, vehicle head lamp control systems, vehicle temperature sensing and display systems, and vehicle tire pressure sensing and display systems.
Vehicle position identification systems are known and commonly used in vehicles for purposes relating to vehicle navigation and tracking systems. Currently, two such position identification systems that are in use are GPS and GLONASS, both of which utilize a constellation of satellites that transmit microwave signals towards the earth that, in turn, are received by a ground-based microwave receiver and used to determine the position of the receiver on the earth's surface. Such systems are capable of a very high degree of accuracy. As a result, a great deal of research has been conducted to construct navigation systems that may be readily incorporated into a vehicle.
Position identification systems have also been used in vehicles with respect to communication systems, particularly emergency communication systems, whereby a vehicle occupant making an emergency call using a cellular telephone need not actually know the vehicle's exact location in order to have emergency vehicles dispatch to that location. Examples of such systems include the ONSTAR.RTM. system from General Motors Corporation and the AUTOLINK.RTM. system available from Johnson Controls, Inc. Other uses of position identification systems in vehicles include the use of position information to identify the time zone that the vehicle is currently in, and the use of such position data to determine which zone of magnetic variance the vehicle is in for purposes of calibrating an in-vehicle electronic compass. See U.S. Pat. Nos. 5,724,316 and 5,761,094, respectively.
Despite all the research that has been conducted and all the literature that has been generated relating to the use of position identification systems in vehicular applications, little consideration had been given to the practicalities of where to mount the microwave antenna that is to receive the microwave signals from the satellites. Published International Application No. WO 97/21127 discloses the mounting of two separate microwave antennas in the two external rearview mirror housings of the vehicle. While there are two microwave antennas located in the external rearview mirror housings, the system receiver circuit is located in the interior of the vehicle. The separation of the receiver circuit from the antennas introduces significant manufacturing difficulties. Coaxial cable typically used to connect the antenna to the receiver is expensive and difficult to handle in a manufacturing process, since it cannot be kinked and is relatively difficult to terminate. Furthermore, such coaxial cable typically has a relatively expensive push-on or screw-on type connectors that connect it to the system receiver circuit and/or microwave antenna. Additionally, vehicle manufacturers have expressed an unwillingness to require their assembly line workers to connect the components using such a coaxial connector.
Locating a microwave antenna in the external rearview mirror housings is also disadvantageous because of the likelihood that dirt, moisture, snow, and humid air may readily reach the microwave antenna and adversely affect its performance. Also, because the reception of microwave signals by the microwave antennas is adversely affected by any metallic or other electrically conductive materials that may exist between the satellites and the antenna, it is necessary to utilize two separate antennas to allow for a sufficient field of view of the satellites so as to accurately determine the vehicle's position. Obviously, the need for this additional antenna significantly adds to the cost of implementing such a system, particularly when one takes into account the need to run two separate coaxial cables to the system receiver circuit. Further, even with a separate antenna mounted in each of the two exterior rearview mirrors, the overall field of view of the system is still restricted by the sides and roof of the vehicle.
While WO 97/21127 further suggests that the antenna could additionally be positioned within the bezel of an interior mirror of the vehicle, doing so is not preferred because the interior mirror bezel is movable with respect to the passenger compartment, which may introduce error in the vehicle position measurements. Further, WO 97/21127 additionally states that metallic coatings on the vehicle windshield may interfere with the operation of a receiving antenna when mounted in an interior rearview mirror assembly. Additionally, like the configuration where the receiving antennas are mounted in the two exterior mirrors, the mounting of the receiving antenna in the interior rearview mirror bezel also presents manufacturing problems associated in connecting the antenna with the receiver, which apparently is mounted in the vehicle instrument panel.